Display apparatus

Abstract

Display apparatus includes reflective layer with reflective material with stacks of additional layers thereon. Each stack has an optically switchable layer. Switching elements are on a side of the reflective layer opposite to the stacks or form part of the reflective layer. Each switching element applies heating to a switchable portion of the optically switchable layer to change appearance of the switchable portion when viewed from the viewing side of the display apparatus. The optically switchable layer includes phase change material switchable between stable states each having a different refractive index. The phase change material switches by applying heat between the stable states. Switching the optically switchable layer causes the apparatus to provide one or both of the following effects for incident radiation within a predetermined frequency range: (i) a change in reflectivity of a factor of at least 50; or (ii) a change in phase within 5% of nπ/ 2 radians, where n is an integer.

1 . A display apparatus, comprising:
 a reflective layer comprising reflective material, and one or more stacks of additional layers provided on a first side of the reflective layer, wherein each stack comprises an optically switchable layer;   a plurality of switching elements located on a second side of the reflective layer opposite to the first side or forming part of the reflective layer, each switching element being operable to apply heating to a switchable portion of the optically switchable layer and thereby change an appearance of the switchable portion when viewed from a viewing side of the display apparatus, wherein   the apparatus is configured to apply the heating by driving an electrical current through the switching element to generate Joule heating in the switching element, wherein the electrical current flows in an electrical circuit comprising a portion of the reflective layer.   
     
     
         2 . The apparatus of  claim 1 , wherein each switching element comprises a resistive component in series with the portion of the reflective layer. 
     
     
         3 . The apparatus of  claim 2 , wherein the resistive component is positioned behind the reflective layer when viewed from the viewing side of the display apparatus. 
     
     
         4 . The apparatus of  claim 2 , configured such that when an electrical current is being driven through the resistive component to drive the switching element, Joule heating associated with the electrical current is higher in the resistive component than in the reflective layer. 
     
     
         5 . The apparatus of  claim 1 , wherein the reflective layer comprises a pattern of regions of different thermal conductivity, the pattern being such as to increase the thermal resistance between each region of the reflective layer that corresponds to a switching element and each region of the reflective layer that corresponds to a different switching element. 
     
     
         6 . The apparatus of  claim 5 , wherein each region corresponding to a switching element comprises an inner sub-region surrounded by a thermally insulating region that surrounds only that inner sub-region, wherein the average thermal conductivity of the material or materials forming the inner sub-region is higher than the average thermal conductivity of the material or materials forming the thermally insulating region surrounding the inner sub-region. 
     
     
         7 . The apparatus of  claim 5 , wherein the inner sub-region predominantly comprises a metallic material and the thermally insulating region predominantly comprises a non-metallic material having a lower thermal conductivity than the metallic material. 
     
     
         8 . The apparatus of  claim 1 , wherein the reflective layer is patterned so that each switching element comprises a localized path of high electrical resistance formed within the reflective layer, the localized path being such that at least 25% of the Joule heating provided by the switching element is provided by Joule heating within the localized path. 
     
     
         9 . The apparatus of  claim 1 , wherein the optically switchable layer is configured such that switching of the optically switchable layer causes the apparatus to provide one or both of the following effects for incident radiation within a predetermined frequency range:
 i) a change in reflectivity of a factor of at least 50; or   ii) a change in phase that is within 5% of nπ/2 radians, where n is an integer.   
     
     
         10 . The apparatus of  claim 9 , wherein the predetermined frequency range has a bandwidth of less than 100 nm. 
     
     
         11 . The apparatus of  claim 10 , wherein the effect (i) and effect (ii) are not provided for incident radiation outside of the predetermined frequency range. 
     
     
         12 . The apparatus of  claim 1 , wherein the optically switchable layer comprises a phase change material, the phase change material being switchable between a plurality of stable states, each of the stable states having a different refractive index relative to each of the other stable states. 
     
     
         13 . The apparatus of  claim 12 , wherein the phase change material is switchable by application of heat between the plurality of stable states. 
     
     
         14 . The apparatus of  claim 1 , further comprising an integrated circuit layer formed on the second side of the reflective layer opposite to the one or more stacks, the integrated circuit layer comprising at least one integrated circuit chip that comprises one or more of: (i) all or a portion of each of a plurality of the switching elements; and (ii) electronics for driving a plurality of the switching elements. 
     
     
         15 . The apparatus of  claim 1 , wherein the reflective layer is metallic. 
     
     
         16 . The apparatus of  claim 1 , wherein each of the one or more stacks of additional layers further comprises a spacer layer, each spacer layer being provided between the reflective layer and the optically switchable layer that is in the same stack as the spacer layer. 
     
     
         17 . The apparatus of  claim 1 , wherein each of the one or more stacks of additional layers further comprises a capping layer, wherein the optically switchable layer in each stack is provided between the capping layer and the reflective layer. 
     
     
         18 . A spatial light modulator comprising the apparatus of  claim 1 . 
     
     
         19 . A projection system comprising the spatial light modulator of  claim 18 . 
     
     
         20 . A security document comprising the apparatus of  claim 1 .